The invention relates to the field of electron beam lithography. More particularly, the present invention relates to the formation of nanostructures using nanocrystals forming shadowmasks for masking electron beam radiation during electron beam lithography.
Low dimensional semiconductor nanostructures are used in electronic, optoelectronic and magnetoelectronic devices. To fabricate semiconductor structures in the nanometer range, it is necessary to develop lithographic techniques with nanometer-scale resolution. Much effort has been made in fabricating nanometer-scale structures using high-energy, highly defined sources such as electron beams and X-rays. The proximity effect, caused by the high-energy sources, limits the resolution of lithography. Scanning techniques with a low energy source, such as scanning tunneling microscope lithography, are not suitable for high throughput. Self-assembly techniques may be utilized to produce shadowmasks that are characterized by periodic, nanometer-scale patterns. Block co-polymers, for example, will phase separate into structures with nanometer-scale periodicities. However, these organic materials result in poor contrasting shadowmasks. Another option is to use higher-contrast inorganic systems. Organically functionalized metal and semiconductor nanocrystals in the two to one hundred nanometer range can assemble into a variety of organized structures, including lamellar wire-like phases. These structures usually consist of self-oriented high-aspect ratio nanocrystals that can be transferred as a Langmuir-Schaeffer film during a Langmuir-Blodgett lift-off process of the film onto substrates of virtually any size. The Langmuir-Blodgett lift-off process has been used to create nanostructures on a substrate bonded to a transfer tool. The Langmuir-Blodgett lift-off process has not been used to create nano sized semiconductor devices.
Nanometer sized film structures, such as strands and dots have been created. These strands and dots have been used for testing predictions of quantum confinement and reduced dimensionality as potential building blocks for nanostructure materials. Nanostructure materials have not been used to create nano size semiconductor devices in the electronics industry. In particular, semimetallic bismuth with very small effective mass and high carrier mobilities is a suitable material for studying quantum-confinement effects in one-dimensional systems and is a promising material for thermoelectric applications. However, semimetallic and silicon based materials have not used to form semiconductor devices on substrates during convention photolithographic processing. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a method for creating nano size semiconductor structures.
Another object of the invention is to provide a method creating nano size electron beam lithography shadowmasks.
Yet another object of the invention is to provide a method creating nano size etch masks using electron beam lithography.
The method is used for fabricating precisely defined nanometer scale photoresist patterns and semiconductor devices. The Langmuir process is utilized to form high aspect ratio lamellae or wire-patterns of silver nanocrystals on the surface of water. The patterns are transferred onto resist-coated substrates as a Langmuir-Schaeffer for producing a shadowmask. The nanostructure patterns are transferred to the photoresist material by spatially selective electron beam exposure on the silver nanocrystal nanostructure shadowmask. The invention forms nanocrystal structures as resist shadowmasks having a predetermined patterns for blocking electron beam exposures. The combined use of low energy electron beam exposure and self-assembled nanocrystal shadowmasks provide a low-cost fabrication technique for forming semiconductor nanometer scale nanostructures.
The low cost and high resolution exposure shadowmask is suitable for low energy electron beam lithography. Nanocrystal assembled dot and wire nanostructures can be used as a shadowmask during low energy electron beam lithography. Oriented nanometer-size dot and strands nanostructures are readily transferred onto a wafer that may be several inches in diameter. Nanocrystal based nanostructures including strands and dots are useful in the processing of nano sized semiconductor devices. The method generates oriented continuous self-assembled nanostructures of semiconductor or metallic materials for used in semiconductor device fabrication. Nanocrystals linked with ligands, such as thiol tails, are fabricated using an organically functionalized nanocrystal solution. Arrays of dots and strands form spontaneously on the surface of water in a Langmuir-Blodgett trough when the nanocrystals in solution are dropped onto the surface of the water in the trough. The nanocrystal structures are then transferred onto electron-beam sensitive photoresist coated substrates by the Langmuir-Blodgett lift-off process, with the modification of firstly depositing an electron beam resist on the substrate. To prepare a thick nanocrystal shadowmask, multiple layers of nanocrystals are added to the previously deposited layer to increase the size of the nanostructure. The nanocrystal patterns are transferred to the photoresist film during electron beam exposure. Spatially selective electron beam exposure on the nanocrystal shadowmask serves to selectively expose the resist layer. Developing the exposed resist layer results in a resist patterned etch mask suitable for further processing during the fabrication of nano sized semiconductor devices. Using the nanocrystal shadowmask, a 50 nm size polymethyl methacrylate (PPMA) nano sized etch mask can be produced for creating a similarly sized device such as a quantum wire or nano wire using reactive ion etching. Very small 15 nm size PMMA dot patterns can also be formed by the method. The PMMA resist pattern etch mask is obtained using the nanocrystal shadowmask. The etch mask is used masking the reactive ion etching or other processes step during further processing of the substrate. The resist etch mask can be used to pattern the substrates, for example, a silicon substrate etch by a subsequent anisotropic reactive ion etching process that is used to form nanowires under the etch mask.
Hence, the nanocrystal strand nanostructure form predetermined patterns that are effectively transferred onto the PMMA coated silicon wafer for patterning the substrate with nanometer scale resolution. The low energy electron beam exposes negative portions of the nanocrystal shadowmask on the resist layer on the substrate. After exposing the resist layer, the resist is then developed for removing exposed portions of the resist to expose a portion of the substrate for reactive ion etching. The method allows for the creation of nano sized structures in a semiconductor substrate for forming nano sized devices. The method uses low energy electron beam exposure that reduces the proximity effects. The method is suitable for low cost and high throughput fabrication of semiconductor nanometer scaled structures and devices. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.